Not Applicable.
1. Field of Invention
The present invention relates to an apparatus and method for the measurement of inductance. More precisely the present invention relates to an apparatus and method for the measurement of inductance of a wire-loop sensor in the presence of a vehicle moving in a traffic lane.
2. Description of the Related Art
It is well known in the prior art to measure the inductance of a wire-loop, which is part of the frequency determining circuit of an LCR oscillator, using frequency-counting techniques. Typically, the number of zero-crossings per time increment of the voltage across the terminals of the LCR capacitor, C, is counted. Because the frequency of the LCR oscillator is inversely proportional to the square root of the inductance, L, of the LCR circuit, changes in the inductance of the wire-loop are reflected in changes of the number of zero-crossings counted per time increment. The Class-C wire-loop oscillator described in U.S. Pat. No. 3,873,964 issued to Thomas R. Potter on Mar. 25, 1975 is typical of LCR oscillators used in the prior-art.
Another problem associated with the measurement of inductance in a wire-loop is crosstalk.
An apparatus for measuring the inductance of a wire-loop with noise-cancellation, auto-calibration and wireless communication features, or detector circuit is shown and described. The apparatus measures the effective change in inductance induced in a wire-loop as a vehicle passes over the wire-loop to produce an inductive signature corresponding to a vehicle.
Generally, the detector circuit includes at least one wire-loop sensor connected to a resistance-capacitance (RC) network to form a fixed-frequency RLC driver circuit. The RLC circuit is coupled to a variable-gain differential preamplifier that buffers and amplifies the differential output of the RLC circuit. The preamplifier is coupled to a demodulation circuit, which mixes the component outputs of the RLC circuit with the output of a demodulation oscillator and generates a demodulated signal corresponding to the envelope of the combined RLC waveform. The demodulation circuit feeds a low-pass filter that removes out-of-band noise and produces a filtered signal. A variable-gain amplification stage amplifies the filtered signal. In order to obtain sufficient amplification and maintain the amplified signal within the bounds of a single power supply, a signal conditioning stage removes a DC offset, which is produced by a DC offset generator, from the filtered signal prior to the amplification stage. An analog-to-digital converter (ADC) samples the amplified output to produce a digitized output of the measured inductance, which represents the inductive signature of the vehicle. When used with wire-loop sensors of appropriate design, the repeatable inductive signatures produced by the detector circuit provide information about the speed and volume of vehicular traffic, the occupancy of the wire-loops sensors and allows classification and re-identification with greater precision and accuracy than is available with conventional detector circuitry. The ability to classify with high precision and accuracy and to re-identify vehicles crossing other wire-loop sensors within a vehicle detection system network allows the determination of travel time and origin/destination information, as well as traffic safety information, such as collision warnings and accident avoidance information.
The operation of the detector circuit of the present invention resembles a frequency modulation-to-amplitude modulation (FM-to-AM) detector circuit, also known as a slope detector circuit, which is used in radio communications. In the detector circuit of the present invention, the frequency of the input signal remains fixed and the resonant frequency of the tuned RLC circuit changes. The change in resonance results from variations in the inductance of the wire-loop, which modulates the amplitude and the phase of the fixed-frequency input. In other words, the input signal is a carrier that is modulated by the vehicle signature.
One method for detecting a vehicle using the detector circuit of the present invention involves monitoring the output voltage of the detector circuit, as compared to frequency counting techniques common in the prior art. An examination of the envelope of the amplitude-modulated waveform provides the desired output voltage information.
Demodulating the amplitude-modulated (AM) waveform produces the envelope of the waveform. When the carrier frequency lies near the resonant frequency of the RLC network, the RLC network attenuates the input signal at the harmonics of the demodulation square wave and also the undesired effects of mixing with a square wave are minimal. A low-pass filter applied to the envelope rejects signals outside of the baseband, which now contains the vehicle signature. The fixed-frequency input is set to a frequency on the skirt of the RLC transfer function on either side of the resonant frequency. This maximizes the amplitude of the resulting inductive signature. The skirt is also fairly linear. Placing the input frequency on one side results in relative signatures that are substantially the negative of signatures produced on the other side of the skirt.
Inductance measurement circuits are susceptible to two types of noise. One is common-mode noise and the other is differential noise, both of which are induced in the wire-loop from ambient sources, such as high voltage lines. The present invention incorporates a number of noise rejection features, which improves the overall performance and efficiency of the detector circuit. By design, the detector circuit of the present invention is double-ended and balanced. Because the signal of interest is differential, subtracting the signal of one leg of the detector circuit from the signal of the other leg rejects common-mode noise. The optional coupling transformer rejects common-mode signals from the wire-loop. In addition, the differential input of the ADC provides another opportunity for common-mode rejection.
The synchronous demodulator of the present invention takes advantage of the differential output from the RLC circuit. Because the output on one leg of the RLC circuit is 180 degrees out of phase with the output on the other leg, switching between the two legs using the switches of the synchronous demodulator is similar to inverting the output signal of the RLC circuit at every other half cycle of the demodulator frequency. This maintains single-supply operation and does not require a multiplication or inversion operation. Overall, this method modulates differential signals while passing common-mode signals. Differential signals outside of the frequency band of interest are rejected while all differential signals inside of the frequency band of interest are kept. The frequency band of interest is selected to be a band that contains a minimum of unwanted signals, for example power line interference, or a band that contains signals that are controllable, such as crosstalk between loop sensors.
An inductive wire-loop is also susceptible to crosstalk. By controlling the frequency of the excitation sources of two or more cross-talking wire-loops to a high precision and with a modicum of coordination, the beat frequency caused by crosstalk between the wire-loops is controlled. Each detector circuit is provided with a unique carrier frequency and distinct frequency band within which to operate. The carrier frequencies need to be spaced far enough apart in order to give enough bandwidth for the signature""s signal. The exact amount of separation between carrier frequencies depends on the number of detector circuits operating in close proximity. The bandwidth required for a signature is mainly a function of vehicle speed, vehicle features and loop geometry.
Inductive loops that are in close proximity to each other are magnetically coupled. One consequence of this coupling is that if one loop is driven by a time-varying voltage causing a time-varying current to flow, part of the magnetic field created by that current will intersect the other loop causing a time-varying current to flow in the other loop. This is a mechanism by which information can be transmitted by one loop and received by another, without requiring the detector circuits to be otherwise physically linked.
The signal created in the receiving loop by the magnetic coupling will be added to the driving signal of the receiving loop that is used to detect vehicle signatures. If the frequency of the communication signal generated by the transmitting loop is different from the frequency of the receiving loop""s own driving voltage and if the bandwidth of the data transmission is low enough, when the two signals are added in the receiving loop, they can be later separated by a processor employing signal processing techniques, and both loops can detect vehicle signatures while simultaneously sending and receiving data.